1. Field of the Invention
This invention relates to an optical transmission apparatus used for NRZ modulation type very high speed optical communication or the like, and more particularly to an optical transmission apparatus having improved clock transmitting/extracting means.
2. Description of the Related Art
In recent years, the optical communication technique has been positively put to practice and enhanced. From the viewpoint of efficient utilization of the same physical resources, an increase in the clock frequency has been strongly required. In order to meet the above requirement, the operation frequencies of various devices constructing the system have been set to values near the respective maximum permissible frequencies. Therefore, the performance of a clock extracting circuit which requires the widest bandwidth in the system is dealt with as one of the most important technical key points.
In the amplitude modulation optical communication in the ultra high frequency band higher than several Gb/s, the NRZ modulation system in which the effective bandwidth can be made narrow is used. An NRZ modulation signal transmitted in the above system is created as follows. That is, it is determined according to a data signal input in synchronism with the clock frequency whether the level of an NRZ modulation signal to be created is kept at the level ("HIGH" or "LOW") of the NRZ modulation signal set in a cycle presented one clock before the present cycle or changed to an inverted level (from "HIGH" to "LOW" or from "LOW" to "HIGH"). Then, this operation is sequentially repeated.
Thus, a transmission signal does not explicitly contain a clock frequency component. Therefore, in the optical communication of a conventional NRZ modulation system, it is necessary to extract a clock component implicitly contained in the reception signal on the reception side and restore the clock based on the extracted clock component. A circuit having the above function is disclosed in an article by MIZUHARA, O., et al, "7.7 Gbit/s BENCHTOP REGENERATOR", ELECTRONICS LETTERS, 1989, Vol. 25, No. 21, pp 1465-1466, for example. In the above circuit, a reception signal whose D.C. component is cut off by a wide band A.C. amplifier on the reception side is rectified by a full-wave rectifier circuit to regenerate a clock frequency component. Then, the clock frequency component is amplified by an amplifier and supplied to a band-pass filter having a large Q so as to suppress the other frequency component except the clock frequency. Further, the clock signal is output via a waveform shaping circuit constructed by a limiter amplifier and a phase shifter so as to extract a clock signal synchronized with the reception signal.
However, this type of method has the following problem. That is, in spite of the fact that the bandwidth of a signal transmitted by use of the NRZ modulation System is suppressed to half of the clock frequency, it is necessary to use an active device having a wider bandwidth than that of the clock frequency on the reception side although only in the partial portion.
Further, with the above method, even when the probabilities of "high" and "low" in the pattern of a signal transmitted are set as close as possible to each other by introducing a signal such as scrambler, the degree of equalization is limited. Therefore, the phase of a clock signal extracted according to a variation in the above pattern would vary. As a result, clock jitter may occur and the clock synchronizing operation of the system will become unstable. Further, since a signal processing operation such as insertion or elimination of the scrambler signal which may lower the transmission efficiency is indispensable, a complicated system configuration is required.
Further, another problem occurs in the application thereof. That is, in recent years, the optical communication is not only used for pure tele-communication but is also used as an optical link for connecting the buses of computers to each other by combining parallel/serial signal converters. In the case of such an application, the data signal itself contains an extremely low frequency component. Therefore, it becomes important to use a means capable of easily and continuously transmitting a clock without processing the data signal.
Thus, in the prior art, in the optical transmission system using the NRZ modulation system, complicated process and circuit construction are required and parts which are operated at a frequency higher than the clock frequency are indispensable. Further, an influence by the transmission signal pattern cannot be eliminated and it is difficult to ensure that the clock can be always stably extracted.